Plastic Forming And Folding/Deploying Behavior Of Transformable Ultrathin Metal Structure

Transformable structure can be used to manufacture large size parts in aerospace because it could be largely deformed without damage.Most of the materials made into transformable structure are polymer at present,wh ile the mechanical properties of metals are bit better than polymer in resistance of hard particle impact,large temperature variation,ultraviolet radiation and shaping in space.Metal foil and thin sheet have smaller thickness and smaller deformation resistance than those of medium and thick metal sheet.The occurance of crack in outer surface and wrinkle in inner surface is difficult for metal foil and thin sheet during bending with small bending radius and large bending angle.In order to make the structure easy to be formed and no local failure occurs during deformation,the structures made of metal foil and thin metal sheet are emplyed.The thickness of transformable metal structure made of these materials have small ratio of thickness to diameter,and they are called as transformable ultrathin metal structure.At present,there is no report for the investigation of transformable ultrathin metal structure in China.In this paper,a type of deformable ultrathin metal structure was designed,and the parametric modeling of the structure was realized by introducing Rhinoceros software.The optimum ranges of important parameters of structure are obtained.The relationship between the important geometric parameters of the structure was analyzed.A double-layer hard paper model with specific geometric parameters was constructed and the deploying characteristics of the structure were evaluated.The variation of surface area during transformation was studied by spatial geometric derivation.The forming method of foils made into ultrathin metal structure is explored and the foil structures are produced.The die is desinged and manufactured by the reversal forming process,and applied to complete the forming of the structures made of 6065 Aluminum,T2 copper,TA1 titanium and SUS304 stainless steel foil,the forming quality of the structures is evaluated.The folding forces are 99 N,138N,255 N and 520 N,and the average height after unloading are 10.2mm,28.1mm,42.8mm,47.2mm respectively for structures with the same geometric parameters,and the materials are 6065 aluminum alloy,TA1 pure titanium,T2 copper and SUS304 stainless steel foil in experiments.The failure mode and mechanism of the structures after multiple passes of folding-flattening process were analyzed,the holes and wrinkles occur in horizental middled surface of the structure at first,which result from the self-coordination of the structure and the excessively reduce of the bending radius in this position.In deploying process of the double-layer structure,force-displacement relationship of double-layer transformable structure made of SUS 304 stainless steel foil was studied experimentally.The results show that the double-layer structure of stainless steel foil exhibits three distinct stages during deploying.The force at first stage increases with displacement linearly;tensile force increases with displacement,reaches maximum value in pirior to decreases slowly in stage 2,indicating the tensile instability in this stage;the bending force increases rapidly with the displacement in stage 3,the necking near the horizental middle surface of the structure disappeares in this stage.The deformation form of self-coordination of the structure plays a dominant role in stage 1,in the second stage th e deformation forms of self-coordination and the deploying at the creases of the structure work synthetically.The circumferential flatten torque play a role in stage 3,the main deformation form is to flatten the structure in circumference.As a basic research on the forming process of transformable sheet metal,the cold bending process of pure titanium sheet and its resistance heating assisted bending process with different current density were systematically studied.8 groups of pure titanium sheets were heated at 0,1.25A/mm~2,2.5A/mm~2,3.75A/mm~2,5.0A/mm~2,6.25A/mm~2,7.50A/mm~2,8.75A/mm~2,respectively,and bent after heating.The heat-transfer of workpiece to mold and environment,and their effects on springback during hot bending were investigated numerically and experimentally.The relationship between force and displacement of TA1 sheet during cold bending after resistance heating was experimentally investigated and compared with cold forming.As a supplementary part of springback research,for a new kind of metal material that could be used to manufacture transformable ultrathin metal structure,namely SUS304 stainless steel,numerical simulation and experimental research of cold and hot bending process of SUS304 stainless steel sheet were conducted.The procedures of resistance heating with different current density,hot bending and springback after unloading are included.The springback and the formation mechanism of 0 springback of sheet with different current density after hot bending are analyzed.It is considered that the negative bending in the bending bottom zone and the straight wall zone promotes the formation of the 0 spring back.In addition,the folding/unfolding of the transformable sheet structure involves the bending/ flattening deformation of the material.Therefore the twinning /detwining behaviour of three bending zones(the outer bending layer,neutral layer and the inner bending layer)and texture evolution along the crystal orientation and the characteristic direction of pure titanium sheet samples were investigated in bending and flattening process.